As described in U.S. Pat. No. 3,902,130 entitled "Improved Apparatus for Lengthening Laser Output Pulse Duration," issued to Charles T. Pike, Aug. 26, 1975, assigned to the assignee hereof, and incorporated herein by reference, recently developed tunable dye lasers permit the generation of spectrally pure laser radiation at very nearly any selected frequency over a range of frequencies associated with the particular dye, rather than being limited to the frequency output of only a few widely separated discrete wavelengths. When this type of laser is used to produce atomic or molecular resonant light, a single frequency of a specific wavelength radiation is typically employed to produce appropriate resonant response.
Where a dye laser is employed for this purpose, it is advantageous to amplify the pulse and increase the pulse duration in order to make more energy available. The Pike patent addresses this problem and solves it as follows. A low-power, short duration and spectrally pure laser pulse is applied to a regenerative laser amplifier. The laser amplifier, when energized or pumped, radiates in a spectrally broad frequency pattern which may be 100 angstroms in width. The broadband radiation is a result of stimulated emission in the active medium of the laser amplifier. If at the start of the stimulated emission, light of a given frequency from a master oscillator is introduced, the stimulated emission will occur only at this frequency. When this occurs, the laser amplifier is said to be locked to the master oscillator. In order for this locking to occur, the master oscillator pulse must exist in the amplifier material at the right instant of time. If the pulse occus early, it will have no effect. If it occurs late, the stimulated emission will have started and the amplifier will have a broadband output when it lases.
Such a system is now known as an injection-locked laser. This type of laser in general involves a master oscillator which is of low power and an injection-locked oscillator (ILO) which acts as the regenerative laser amplifier and includes an active medium into which the signal from the master oscillator is injected.
In order to time the generation of the pulse from the master oscillator so it occurs at the moment when the laser amplifier has been excited to an amplifying condition, but before self-oscillation, a control circuit is provided to activate the master oscillator in timed relationship to the pumping of the laser amplifier. This commences the regenerative cycle that produces a sequence of output pulses.
While the system described in the Pike patent operates very well for allowing the production of the high-power pulses of appropriate spectral content, degradation over time of the laser components may cause the production of a pulse from the master oscillator at a time which is non-optimal with respect to the energization of the active medium and a broad spectral output results. When this system is utilized for isotope separation, this broad spectral output may destroy the isotope selecting mechanism.
With respect to laser isotope separation processes, the master oscillator beam carries the precise narrow bandwidth wavelength information necessary for proper operation of the laser isotope separation process, and the ILO increases the energy in each pulse to the level required by the process. If the ILO fails to lock to the master wavelength, the energy contained in the pulse will be spread over a band-width several orders of magnitude greater than that needed to match the absorption bandwidth of the isotope to be separated, drastically reducing process efficiency. In fact, if the bandwidth is too broad, ionization of both the desired and undesired isotopes may occur, which defeats the separation process.
In summary, whether the ILO locks to the master oscillator pulse depends on the time of arrival of the master oscillator pulse with respect to the triggering of an ILO flashlamp and also the peak power of the master oscillator pulse. Lock can be lost due to the aging of the triggering circuits, Thyratron switches normally utilized, or the flashlamp. This problem is especially severe if the master oscillator pulse power is just great enough to lock when timing is optimally set.
If the master oscillator pulse arrives too early, the population inversion in the ILO active medium is too low to initiate stimulated emission at the master oscillator wavelength and hence lock to it. If the pulse arrives after the ILO has begun laser action on its own, the output will again be the broadened characteristic of the ILO cavity and the output will not be locked.